Custom CSS file referencing images

Hello,

I'm new to design studio and I've been reading tutorials and also went through some documentation about CSS.

I could add a custom CSS file to my design studio app.

But I dont know how to implement a CSS style like this:

.green_marble {

  background: url(./my_green_marble.png) no-repeat ....

}

I dont know where to place the png image, I paced it in the same place as the css with no luck (I can use other css styles within this CSS file so this means the file is loaded correctly).

I will appreciate any help on this.

Thanks